David Crouch and Helena Bengtsson “Stockholm truck drives into crowd, killing three people” https://www.theguardian.com/world/2017/apr/07/truck-crashes-in-central-stockholm-sweden
In recent years, several hundred young Swedes have travelled to Syria to join Isis, making Sweden one of the main sources of European recruits to the Islamist group.
If the event is confirmed to be a terrorist attack, it will be the first in Sweden since an Iraqi-born Swedish man*3 exploded two bombs in Stockholm in 2010, one of which killed him. No one else was injured.
*3:See eg. Jonathan Paige “Stockholm suicide bomber: Taimour Abdulwahab al-Abdaly profile” https://www.theguardian.com/world/2010/dec/12/stockholm-suicide-bomber-profile